A parallel mechanism is a manipulator that is capable of operating at high speed with high precision and is lightweight and highly rigid, as compared to a serial mechanism employing serial arms. Related art for such a parallel mechanism is described in Patent documents 1 and 2 shown below. Parallel mechanisms described in these patent documents have three arms in total. Each arm has a pair of rods arranged parallel to one another. A motor is connected to a base end portion of the arm in such a manner that power can be transmitted, and a bracket is attached to a free end portion of the arm. An end effecter can be attached to the bracket. For example, a hand can be used as the end effecter.
The motor rotates to operate each arm, and thereby move the end effecter to a given position. Here, the rods and the bracket of each arm are connected via ball joints, and therefore the bracket can move in three dimensions with respect to the arm. Also, as shown in FIG. 5 of Patent document 1, a pair of rods is configured so as to be pulled closer to each other by a spring, and thus a ball portion and a socket portion of the ball joint are connected so as not to be disengaged from each other. In this manner, when the motor rotates to operate the arm, the bracket is not disengaged from each rod.
In the foregoing related art, each hook provided at opposite ends of the spring is simply hooked on each rod. The spring hook has poor durability, and therefore when the arm is continually operated, the hook might be broken due to fatigue. Also, in this configuration, since the spring hook is simply hooked on the rod, there is scope for the rod to rotate about an axis that is parallel to its longitudinal direction. Accordingly, when operating the arm, the rods are inevitably caused to rotate about the axis, resulting in an increased amount of movement of the socket portion with respect to the ball portion at the ball joint. Therefore, the ball joints, which are made of synthetic resin, deteriorate quickly due to abrasion of the ball portion and the socket portion, and such abrasion of the ball joints leads to unstable attitude control of the end effecter (bracket). This results in a problem in that the end effecter is not positioned with high precision. Patent document 3 also describes a manipulator structure. This structure addresses the problem of the spring hook being broken due to fatigue, but it does not address any problem caused by rotation of the rod about the axis that is parallel to a longitudinal direction of the rod.    Patent document 1: Japanese Utility Model Application Laid-Open Publication No. H8-403    Patent document 2: Japanese Patent No. 3050992    Patent document 3: Japanese Unexamined Patent Application Publication No. 2002-529258